The EFG process is well known, as evidenced by the following U.S. Pat. Nos. 4,230,674; 4,661,324; 4,647,437; 4,968,380; 5,037,622; 5,098,229; 5,106,763; 5,156,978; and 5,558,712. In the EFG process, crystalline bodies having a predetermined cross-sectional shape, e.g., tubes of octagonal cross-sectional shape, are grown on a seed from the liquid film of a selected material which is transported by capillary action from a melt contained in a crucible to the top end surface of an EFG die. In order to grow relatively large tubes, e.g., tubes of octagonal cross-sectional shape measuring 4–6 inches on each flat side and 15–20 feet in length, it is necessary to replenish the melt during the growth process, with the replenishment being conducted so as to keep the level of the molten silicon substantially constant in the crucible containing the melt. The silicon feedstock used in growing silicon tubes is typically in the form of substantially spherical pellets (“shot”) having a diameter in the order of 2 mm. The common practice is to deliver additional feedstock to the crucible on an intermittent basis according to the rate of consumption of the melt, so as to maintain the level of the melt in the crucible within predetermined limits.
The EFG process has been used extensively to grow silicon tubes, e.g., tubes of octagonal and nonagonal shape, and those tubes are subdivided by cutting out square or rectangular wafers from their flat sides. Those wafers are then used as substrates to form individual photovoltaic cells. U.S. Pat. Nos. 4,751,191, 5,106,763, 5,270,248 and 5,320,684 illustrate methods used to manufacture silicon solar cells from wafers cut out of EFG-grown tubes.
The EFG wafers used for making solar cells are p-type silicon having a bulk resistivity in the range of 2 to 4 ohm-cm, with the p-type conductivity and desired resistivity being due to the presence of boron dopant in the melt. The ratio of silicon to dopant atoms required for producing a bulk resistivity of 2 to 4 ohm-cm is very large, approximately eight orders of magnitude. This requires careful control of the quantity and method of introduction of dopant into the silicon melt. For years doping the melt was achieved by adding a small, carefully measured, amount of silicon pellets (also referred to as “shot”) highly doped with boron to a predetermined quantity of intrinsic (pure) silicon feedstock (also in the form of pellets), with the amount of highly doped silicon pellets being set to achieve the doping level required to grow tubes of desired resistivity. The mixture of doped silicon shot and pure silicon feedstock was intermixed and dissolved to form a melt, with the boron in the doped silicon shot being dissolved uniformly throughout the melt.
However, highly doped silicon shot is expensive and difficult to obtain. For these reasons, it was deemed necessary to find an alternative and less expensive method of providing boron-doped silicon shot suitable for doping a silicon melt. One such method is described in my copending U.S. patent application Ser. No. 10/142,312, filed 9 May 2002 (Publication No. U.S. 2003/0209188 A1, published 13 Nov. 2003). That method comprises (1) immersing intrinsic silicon shot in a spin-on dopant solution that consists essentially of a borosilicate in a volatile organic solvent, plus a polymer precursor, and (2) removing the solvent so as to leave a polymeric coating or film containing boron on the shot. The boron-coated shot is then intermixed and melted with a measured quantity of intrinsic silicon pellets to provide a boron-doped silicon melt for use in growing silicon tubes of suitable resistivity.
A significant limitation of the foregoing method is that its use of a volatile organic solvent presents an environmental and safety problem.